It is known that the blow molding of a container generates both an axial (or length) stretch and a radial (or hoop) stretch which provides molecular bi-orientation—and hence a good structural rigidity—to the material of the container.
Although the structural rigidity of an ordinary container is generally sufficient in common applications (such as still water) to resist low mechanical constraints due to the hydrostatic pressure of the liquid therein, it is insufficient to withstand high mechanical constraints generated in the material during a hot-filling.
Indeed, residual stresses in the material are released during hot-filling (particularly with a liquid having a temperature higher than the glass transition temperature of the material), causing a deformation of the container that could make it unsuitable for use—and hence for sale.
To decrease deformation of the container during hot-filling, it is known to run the blow molding within a mold cavity usually provided with sidewalls heated at a predetermined temperature and with a mold base, and to complete the blowing through a thermal treatment called heat set, by which the container is held in contact with the sidewalls at a predetermined temperature between 80° C. and 180° C. for a predetermined time (generally several seconds).
Generally, two sidewalls are present. They are usually named “half-molds”. They can be moved away the one from the other and from the mold base for allowing either the introduction of a preform or the removal of a finished container and placed in close contact together and with the mold base during the stretch blow molding step.
However, heat setting solves only part of the problems of deformation of a hot-fill container. Indeed, while cooling, the liquid and the air above the liquid in the capped container undergo a decrease in volume that tends to make the container retract.
Several solutions have been considered for decreasing the visible effects of such retraction. These solutions generally concern the shape of the container.
For example, it has been proposed to equip the body of the container with deformable side panels that bend inwards under the effect of the retraction and bend back outwards when the container is opened. Such containers must be handled with care by the user because of the flexibility of the body, which may result in accidental spraying.
It has also been proposed to provide the container with a base portion capable of withstanding the various stresses and strains applied to the container, see e.g. U.S. Pat. No. 6,896,147 (assigned to Graham).
More recently, it has been proposed to give the bottom of the container a special shape capable of absorbing at least part of the deformation due to retraction whereas the body of the container is provided with a rigid (i.e. resistant to hot-fill deformation) structure, see e.g. U.S. Pat. No. 7,451,886 (assigned to Amcor).
Deformable bottoms take advantage over deformable side panels in that from the user's point of view the container has a more attractive shape. However, the amount of vacuum absorbed by deformation of the deformable bottom may be insufficient and result in a deformation of the body which may take an oval shape (such a well-known deformation is called “ovalization” by the skilled technicians).
U.S. Pat. Appl. No. 2008/0047964 (Denner et al, assigned to CO2PAC) discloses a container comprising a pressure panel located in the bottom portion of the container. The pressure panel is movable between an outwardly-inclined position and an inwardly-inclined position to compensate for a change of pressure inside the container. In order to alleviate all or a portion of the vacuum forces within the container, the pressure panel is moved from the outwardly-inclined position by a mechanical pusher in order to force the pressure panel into the inwardly-inclined position. The inversion of the pressure panel from the outwardly-inclined position to the inwardly-inclined position reduces the internal volume of the container.
Denner also provides an exemplary method of blow molding such a plastic container, which includes enclosing a softened polymer material such as PET within a blow mold having side wall portions and a mold base portion movable with respect of the side mold portions in the vertical direction between a retracted position and an extended position. The mold base portion is displaced upwardly into the mold cavity to form a transverse pressure panel deeply set within the base portion of the container.
Although Denner provides a solution which in theory alleviates the drawbacks of previous solutions with deformable bottoms in that it maximizes the amount of vacuum compensation, Denner fails to disclose the details of the movable mold base portion, the use of which, as may be noted, is already known through U.S. Pat. No. 4,035,455 (Rosenkranz et al). More specifically, Denner fails to point out specific structural features of the mold base which would allow the target container to be correctly formed. In addition, blow molding trials showed that a container according to Denner is not suitably designed for blow molding or for a proper inversion of the pressure panel after filling and capping.